Method of making cadmium sulphide pigments



Patented Nov. 5, 1940 PATENT OFFICE METHOD OF MAKING C'ADMIUM SULPHIDEPIGMENTS James J. OBrien, Baltimore, Md., assignor to The GliddenCompany, Cleveland, Ohio, 21. corporation of Ohio 1 N Drawing.Application January 5, 1939,

Serial No. 249,413

4 Claims.

This invention relates to the manufacture of cadmium sulphide pigmentsand more particularly relates to the production of such pigments usingcadmium sulphate solutions.

In the process of making cadmium sulphide and allied pigments one of theprincipal raw materials is cadmium sulphate. As pure cadmium can beobtained on the market in the form of the metal, at a reasonable figure,whereas the pure sulphate is not readily obtainable, solutions ofcadmium sulphate are generally made by the action of sulphuric acid onthe metal. acid slowly attacks cadmium with evolution of hydrogen, butthe solution at best isslow. The IF hydrogen, of course, represents amore or less serious fire hazard, depending on plant conditions.

I have found that the solution of the cadmium metal can be acceleratedtremendously by the use of some nitric acid with the sulphuric acid. Thenitric acid is reduced during the reaction, a large amount of ammoniabeing formed that is vaporized during the process. The amount of nitricacid used is ordinarily such that cadmium nitrate is not formed, all ofthe metal dissolved being dissolved forming cadmium sulphate. The nitricacid acts as an oxidizing agent, and this probably accounts for thelarge amount of ammonia and other volatile nitrogen compounds formedduring the reaction.

It has been found that a cadmium sulphate solution neutral to methylorange is desirable in the production of cadmium sulphide pigments,better results being obtained when such a solution is used than if anacid solution is used. This neutrality can be produced by treatment ofthe acid solution wtih alkalies, alkali metal carbonates, etc., butsoluble salts are formed in such a case. If these soluble salts arepresent the size of pigment aggregates is increased, and the pig- 40ment has a weaker tinting strength. Cadmium oxide can be used toneutralize, but is expensive.

It has been found that if an excess of metal is used the resultingsolution is neutral to methyl orange, and is excellently suited for theproduction of cadmium sulphide pigments. When the cadmium sulphatesolution is prepared with an excess of metal, better pigment is producedthan when no such excess is used.

As previously pointed out the amount of nitric acid is preferably suchthat cadmium nitrate is not formed. The proportion of nitric tosulphuric acid may, however, vary over a considerable range, and may besuch that either a nitrate'free solution results or one in which somevolatile nitrates are formed, such as disclosed in my copending ap- Hotdilute plication Serial No. 187,665, filed January 29-, 19 38. Forexample, 1 part by weight of 42 Baum nitric acid to 4.37 to 4.55 partsby weight of Baum sulphuric acid, and 1 part by Weight of 38 Baum nitricacid to 3.68 parts of 60 Baum sulphuric acid will give a nitrate freesolution while 1 part by weight of 38 Baum nitric acid to 3.48 parts of60 sulphuric acid gives a solution containing some nitrates, and 1 partby weight of 42 Baum nitric acid to 3.48 parts by weight of 60 sulphuricacid gives a solution high in nitrates. The amount of nitric acid forthe present invention will ordinarily be 10-30% of the sulphuric acid byweight.

Example I 190# of 38 Baum nitric acid and '700# of 66 Baum sulphuricacid (13.6%), were mixed and diluted with 240 gallons of water, 800# ofcadmium metal was then added and heat applied, after the reactionstarted considerable heat was evolved and the solution boiled. Thetemperature was 180 to 24121" F. and the reaction was complete in about4 hours. The resulting solution was neutral to methyl orange and free ofnitrates. The cadmium sulphate solution thus prepared was purified bytreatment with potassium permanganate to remove manganese and. iron andthe solution standardized to 35 Baum. 266 gallons of this cadmiumsulphate solution was then precipitated with 710 gallons of a 155 Baumbarium sulphide solution. The precipitate was filtered, washed, groundand calcined to produce a yellow cadmium lithopone.

Numerous tests were made using varying ratios of nitric acid tosulphuric acid to show the eifect of varying amounts of nitric acid onthe amount of nitrates in the cadmium sulphate solution. In all of thesetests an excess of cadmium metal was used and the cadmium sulphatesolution formed produced better pigment than a solution in which excessmetal was not used. In those cases in which nitrates were present in thecadmium sulphate solution additional improvements in the brightness,tinting strength, etc., of the pigment prepared therefrom resulted, asmore fully pointed out in the above mentioned application Serial No.

187,665, p it being understood that various expedients may be resortedto to provide nitrates during the calcination of the pigment.

Example II A ratio of 1 part by weight of 42 B. nitric acid to 4.35parts by weight of 60 B. sulphuric acid will produce after dilution acadmium sulphate liquor containing a trace of nitrates, amounts ofnitric acid less than that corresponding to this ratio yielding nitratefree solutions, and amounts of nitric acid greater than thatcorresponding to this ratio give increasing amounts of nitrates.

Cadmium metal 1812 grams 421 Be. nitric acid 383 grams B. sulphuric acid1667 grams Water 4:250

Temperature -2 12. F Neutral to methyl orange.

Nitrates A trace.

' will be 10-30% of the sulphuric acid by Weight.

' The invention is applicable to the production of sulphide pigmentsgenerally. Either the socalled pure colors, or the lithopone type colorsmay be made. Other soluble sulphides such as sodium sulphide may be usedin place of barium sulphide. Either the yellow pigment or the red(cadmium sulpho-selenide) cadmium pigments may be made.

Whenever the term excess meta is used in the specification of claims itis used to mean that a 'suificient amount of metal is used such that asolution neutral to methyl orange containing no free acids results afterall the metal is dissolved which is going to be dissolved.

This application is in part a continuation of Serial No. 739,813, filedAugust 14, 1934, and in part a *continuation of Serial No. 187,665,filed January 29, 1938.

I claim:

1. The process of making a cadmium sulphide pigment which comprisesreacting cadmium metal with a mixture of sulphuric acid, nitric acid andwater until all of the S04 ions of the mixture are combined with cadmiumand an acid-free, nitrate-free CdSO4 solution neutral to methyl orangeis formed, the cadmium metal being in excess of the stoichiometric ratioto the S04 of the mixture and the nitric acid being sufiicient to efiectrapid combination with cadmium of all of the S04 content of the mixturebut insufiicient to impart nitrates to the CdSOa solution and reactingthe cadmium sulphate solution thus formed with a 'Water solublesulphide.

2. The process of claim 1 in which the soluble sulphide is bariumsulphide.

3. The process of claim 1 in which the temperature of the reactionbetween the metal and the mixture of acids is from 180 F. to the boilingpoint of the solution.

4. The process of making cadmium sulphide pigments comprising reacting acadmium sulphate solution with a water soluble sulphide, said cadmiumsulphate solution being the product of the reaction of cadmium metalwith a mixture of sulphuric acid, nitric acid and water until all of theS04 ions of the mixture are combined with cadmium and an acid-free,nitrate-free CdSO4 solution neutral to methyl orange is formed, thecadmium metal being in excess of the stoichiometric ratio to the S04 ofthe mixture and the nitric acid being 'sulficient to effect rapidcombination with cadmium of all of the S04 content of the mixture butinsufficient to impart nitrates to the CdSO4 solution.

' JAMES J. OBRIEN.

